1. Field of the Invention
The present invention relates to an apparatus and a method of measuring optical properties of a Fresnel lens, a hybrid lens, a diffractive beam splitter, etc. (hereinafter referred to generically as diffractive optical elements (DOEs)) with high accuracy, in particular, in terms of diffraction efficiency in laser-beam diffraction and intensity uniformity of split beams.
2. Description of the Related Art
In the development of DOEs used in laser processing systems which perform, for example, multipoint simultaneous drilling, it is necessary to accurately evaluate optical properties such as diffraction efficiency and intensity uniformity of split beams of the DOEs, which affect the performance of the processing systems. A known method for evaluating the optical properties will be described below with reference to FIG. 9.
A laser beam having a power of Pin is incident on a DOE 18, which is a test piece, and is split into multiple beams (seven in FIG. 9). The split beams are converged on an image plane at corresponding positions by a lens 19. The diffraction efficiency is defined as the ratio of the sum of the power Pk of each split beam to the incidence power Pin. The diffraction efficiency represents the energy utilization efficiency in the use of the DOE 18.
Normally, the ratio is 0.6 to 0.9 (the diffraction efficiency is 60% to 90%), and the rest 1−η indicates a loss dissipated to the environment as noise. Uniformity of the power Pk of split beams is expressed by the standard deviation σ or the maximum-minimum range R. Equations for calculating the diffraction efficiency η, the standard deviation σ, and the maximum-minimum range R are shown in FIG. 9. In the equations, NS indicates the number of beams into which the laser beam is split (hereinafter referred to as a splitting number), and {overscore (P)}k indicates the average of Pk.
According to the above-described definitions, the diffraction efficiency and the intensity uniformity of the split beams can be calculated if the power Pin of the incident laser is measured with a power meter and the power Pk of each split beam is measured at each focus point where a pinhole aperture of a suitable size is set.
However, the above-described evaluating method has the following problems with regard to the measurement accuracy:
(a) The measurement accuracy depends on the accuracy of power meters.
In order to measure the diffraction efficiency with high accuracy, the power Pin of the incident beam and the power Pk of each split beam must be measured with high absolute accuracy. For example, when the splitting number of the laser beam is large, such as over a hundred, Pk is smaller than Pin by two orders of magnitude or more. Therefore, the absolute accuracy of the power meter is extremely important. In addition, the repetition accuracy is important for measurement of the intensity uniformity of the split beams. However, the absolute accuracy of commercial power meters is normally ±3% to ±5%, and is not sufficient.
(b) Power stability of the laser beam greatly affects the measurement accuracy.
When the laser power is unstable, measurement values of Pin and Pk vary and the measurement accuracy decreases. The power stability of commercial carbon dioxide lasers is normally ±5% to ±10%, and is also not sufficient.
(c) The size of the pinhole for allowing the split beams to pass therethrough affects the measurement results.
In order to accurately measure the power Pk of each split beam, the pinhole size must be optimized. However, since the intensity distribution is widely spread with very low intensity side lobes at each spot, it is extremely difficult to determine the pinhole size. If the pinhole size is too small, the power cannot be sufficiently collected and Pk will be smaller than the actual value, and if the pinhole size is too large, environmental noise and the power of the neighboring split beams will be collected and Pk will be larger than the actual value. Thus, the measurement results vary in accordance with the pinhole size, and sufficient reliability cannot be obtained.
(d) The quality of the laser beam greatly affects the measurement accuracy.
If the size of the focal spots increases because of the transverse mode characteristics and the wavefront aberration of the laser beam and the shape of the focal spots is distorted, it becomes increasingly difficult to determine the pinhole size.
(e) Characteristics of a lens included in the measurement system greatly affect the measurement accuracy.
The power Pk of each split beam is affected by the transmittance of a lens used. When the transmittance decreases, the measured diffraction efficiency decreases accordingly. In addition, the aberration of the lens also distorts the focal spots similarly as the laser quality does. Since off-axis aberrations depend on the incidence angle of the beam onto the lens, the value of Pk decreases as the splitting number increases and the incidence angle increases accordingly.